Leaf color is an important characteristic of normal chloroplast development. Variegated plants have green- and white-sectored leaves, which can be used to identify important pathways and molecular mechanisms of chloroplast development. We studied two Brassica napus variegation mutants from same one variegated ancestor, designated ZY-4 and ZY-8, which have different degrees of variegation. When grown in identical conditions, the ratio of white sectors in ZY-4 leaves is higher than in ZY-8. In both mutants, the cells in green sectors contain normal chloroplasts; while, the cells in white sectors contain abnormal plastids. Seedling chloroplasts ultrastructure of both mutants showed that the biogenesis of chloroplasts was blocked in early stages; delayed development and structual damage in ZY-4 were more serious than in ZY-8. Employing bulked segregant analysis(BSA), two bulks (BY142 and BY137) from BC2F1 lines derived from ZY-4 and ZS11, and one bulk (BY56) from BC2F1 lines derived from ZY-8 and ZS11, and screening by Brassica 60K SNP BeadChip Array, showed the candidate regions localized in chromosome A08 (BY142), C04 (BY137), and A08 (BY56), respectively. Transcriptome analysis of five seedling development stages of ZY-4, ZY-8, and ZS11 showed that photosynthesis, energy metabolism-related pathways and translation-related pathways were important for chloroplast biogenesis. The number of down- or up-regulated genes related to immune system process in ZY-4 was more than in ZY-8. The retrograde signaling pathway was mis-regulated in both mutants. DEG analysis indicated that both mutants showed photooxidative damages. By coupling transcriptome and BSA CHIP analyses, some candidate genes were identified. The gene expression pattern of carotene biosynthesis pathway was disrupted in both mutants. However, histochemical analysis of ROS revealed that there was no excessive accumulation of ROS in ZY-4 and ZY-8. Taken together, our data indicate that the disruption of carotene biosynthetic pathways leads to the variegation phenotypes of ZY-4 and ZY-8 and there are some functions that can compensate for the disruption of carotene biosynthesis in ZY-4 and ZY-8 to reduce ROS and prevent seedling mortality.

中文翻译：

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胡萝卜素生物合成的破坏导致甘蓝型油菜异常质体和杂色的叶子。

叶色是正常叶绿体发育的重要特征。杂色植物有绿色和白色扇形的叶子，可用于识别叶绿体发育的重要途径和分子机制。我们研究了来自同一杂种祖先的两个甘蓝型油菜杂色突变体，命名为ZY-4和ZY-8，它们的杂色程度不同。在相同条件下生长时，ZY-4叶片中白色扇形的比例高于ZY-8。在这两个突变体中，绿色部分的细胞都含有正常的叶绿体。同时，白色区域的细胞含有异常的质体。这两个突变体的幼苗叶绿体超微结构都表明，叶绿体的生物发生在早期被阻止；ZY-4的发育迟缓和结构性损伤比ZY-8严重。采用批量隔离分析（BSA），来自ZY-4和ZS11的BC2F1品系的两个本体（BY142和BY137），来自ZY-8和ZS11的BC2F1品系的一个本体（BY56），并通过Brassica 60K SNP BeadChip进行筛选阵列显示分别位于染色体A08（BY142），C04（BY137）和A08（BY56）的候选区域。对ZY-4，ZY-8和ZS11五个幼苗发育阶段的转录组分析表明，光合作用，能量代谢相关途径和翻译相关途径对叶绿体生物发生很重要。ZY-4中与免疫系统过程相关的下调或上调基因的数量大于ZY-8。在两个突变体中逆行信号通路均被错误调节。DEG分析表明两个突变体均显示出光氧化损伤。通过结合转录组和BSA CHIP分析，确定了一些候选基因。在两个突变体中，胡萝卜素生物合成途径的基因表达模式均被破坏。然而，对ROS的组织化学分析表明，在ZY-4和ZY-8中没有过量的ROS积累。综上所述，我们的数据表明，胡萝卜素生物合成途径的破坏导致ZY-4和ZY-8的杂色表型，并且某些功能可以补偿ZY-4和ZY-8中的胡萝卜素生物合成的破坏，从而降低ROS和防止幼苗死亡。